I’m not sure there is a firm rule but I understand the confusion.
Classification by species would normally dictate that lower case was used, if you think in terms of dog, cat, human, fish etc. The confusion occurs when the category of type coincides with, and is derived from, a place name, Chinese, American, Martian etc. The names of locations are given capitalisation to emphasise their importance, the labels of the inhabitants of those locations inherit the capitalisation.*
This would suggest that human would be lower case but Earthling would be upper, does that make sense? Well it would do apart from where two categories of race are used in proximity, using Martian to denote an inhabitant of Mars demands that equal importance and emphasis should be given to Human.
Where the name of the race isn’t a direct derivative of a location or similarly capitalised origin I believe consistency is the only rule you need to adhere to.
*(This is also true of derivatives from other normally capitalised origins - Christian for example)
[This message has been edited by Crazy Eddie (10-09-2002 02:55 PM).]